This application claims the priority of Korean Patent Application Nos. 10-2005-0089517 and 10-2005-0119285, filed on Sep. 26, 2005 and Dec. 8, 2005, respectively, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an apparatus and method for simultaneous all-optical wavelength conversion and clock signal using semiconductor optical amplifiers (SOAs), and more particularly, to converting wavelengths from non return-to-zero (NRZ) format data based on non-linearities of SOAs and, at the same time, obtaining a pseudo return-to-zero (PRZ) signal from the non return-to-zero (NRZ) and an optical clock signal from the obtained PRZ signal by using time dependent gain-saturation and self-phase modulation (SPM) experienced by the NRZ data while passing through the SOAS.
2. Description of the Related Art
When non-return-to-zero (NRZ) data signals are used in optical communications, it is possible to use hardware with smaller bandwidth in NRZ transmission than in return-to-zero (RZ). Therefore, the costs of parts needed in the systems can be reduced.
However, since NRZ signals have much weaker clock signal components than RZ signals, studies have been investigated to extract optical clock signal components from the NRZ signal.
For example, optical clock signal extraction from an NRZ signal is based on self-phase modulation (SPM) experienced by the NRZ passing through semiconductor optical amplifiers (SOAs).
In general, a leading edge of an NRZ signal amplified by an SOA includes an overshoot that has strong clock signal components.
The overshoot due to self-phase modulation (SPM) is shifted to the red optical wavelengths.
Therefore, a pseudo return-to-zero (PRZ) signal can be extracted from the NRZ by filtering the red-shifted optical wavelengths using a narrow line width filter. (H. J. Lee et al., Electron. Lett. 35 (12), 989 (1999)).
However, a conventional wavelength converter using SOAs extracts a probe beam to obtain a wavelength-converted signal and filter out a pump beam having an overshoot shifted to the red optical wavelengths due to self-phase modulation.
Attempts have been doing to simultaneously perform wavelength conversion and optical clock signal extraction. Wavelength converter can reduce blocking caused by wavelength collisions in a wavelength division multiplexing (WDM) communications network and increase flexibility and capacity of the network by reusing wavelength.
W. Hong et al., IEEE Photon, Technol. Lett., 16, 200 (2004) discloses a non-linear loop mirror having SOAs which convert the wavelength of an NRZ signal and extract a PRZ signal from the NRZ signal.
U.S. Patent No. 2002-0063944 (Donghwan Kim and others) discloses a ring-type semiconductor optical fiber laser converting wavelengths at an ultra-high speed without using an external pump beam and, at the same time, a clock pulse generator producing an optical pulse string phase-locked with input data signal by injection-mode-locking.
In detail, an ultra-high speed optical wavelength converter composed of the ring-type semiconductor optical fiber laser is possible to perform simultaneously wavelength conversion without an external pump beam and extraction all optical phase locking signal.
However, since the ultra-high speed optical wavelength converter uses four wave mixing, it is sensitive to the polarization of an incident signal and is rarely used due to its questionable stability and unreliability.